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As far as Electrum goes, from what I've read, I cannot trust the whole "your wallet can be spawned with a seed" setup. Im too paranoid with the idea that the right combination of words can spawn your entire wallet. It's a bit fucked up actually.
well your bitcoin core wallet is also an HD wallet meaning it is "spawning" all your keypairs from a seed, it just doesn't give you any mnemonic!
as for your paranoia, it all comes down to that random entropy the wallets are using. if it is good enough then the "combination of words" you see is showing that random entropy to you in a human readable form that you could write down.
Replying to you and also to bob123: I know they changed the format from the original wallet.dat after 0.13 or so and then it became an HD wallet too, but in a post I remember achow101 explained some differences in the way the seed is generated and basically said that the way it works is that wallet.dat HD format is safer than Electrum's HD format and you are protected from key derivation attacks. So when I did read that I decided that I will not bother with Electrum and will leave things as they are because im too scared to screw up in the process and then long term being paranoid about Electrum fucking me over. Not to mention all the numerous exploits we've seen lately with the update model.